1. Field of the Invention
The present invention relates to a surface-mount type semiconductor device and, in particular, to a semiconductor device in which, for example, a semiconductor element mounted on a lead frame is sealed with a resin.
2. Description of the Related Art
A resin-sealed type semiconductor device comprises a semiconductor element mounted on a bed of a lead frame, fine metal wires electrically connecting electrodes of the semiconductor element to external leads of the lead frame, a resin layer fixedly sealing the semiconductor element, a portion of the lead frame and the fine metal wires, and a recess formed in the resin layer at a resin-sealing step. The resin sealing step is carried out by locating an assembly of the semiconductor element, a portion of the lead frame and the fine metal wires in the cavity of a mold, introducing a sealing resin which is melted in a pot into the mold cavity, subjecting the resin thus introduced to an aging process, and after completing such a series of steps, knocking a resin-sealed semiconductor element out of the mold by an ejector pin which is located at a lower mold half thereof. Since, at this knock-out step, the sealing resin is not yet solidified, a recess is formed at that place of the sealed resin layer where the resin-sealed semiconductor element is knocked out by the ejector pin. The ejector pin is somewhat projected out of the lower mold half of the mold. In the prior art, there is no proposed concept that the dimension of the ejector pin be controlled. Therefore, the depth of the recess thus obtained in the resin-sealing step varies over a range of 80 to 150 .mu.m.
The semiconductor device, after being temporarily fixed to the mount substrate, has its external lead soldered to the mount substrate. In the conventional sealed resin layer, cracks are developed due to thermal stress upon assembly of the aforementioned components by soldering, so that a decided imperfection occurs due to such cracks. This imperfection involves the following defects:
(1) After the device is mounted as set forth above, the sealed resin layer absorbs moisture through the cracks under a practical atmosphere, causing the semiconductor substrate or associated components to be corroded due to the moisture intruded.
(2) After the device is mounted as set forth above, cracks occur near the recess by thermal load caused at the soldering step and become more wide open there, bringing a "temporary state of fixing" into an unstable state. In an extreme case, the semiconductor element is removed from the bed.
In the surface-mount type semiconductor device, the resin layer is maintained to at least about 1.0 mm. There is, however, a growing demand that a compact semiconductor element be applied at a high integration density to the semiconductor device. A package with a semiconductor element sealed with a resin requires more exact external lead pitch and is made as near a bare chip as possible. However, no adequate preparation has been made so as to solve the aforementioned problems.